Continuously variable permanent magnet for imaging purposes



Nov. 3, 1964 G. w. GOETZE 3,155,860

CONTINUOUSLY VARIABLE PERMANENT MAGNET F OR PURPOSES IMAGING Filed Oct. '7, 1960 2 Sheets-Sheet l Fig. I

WITNESSES INVENTOR b Gelmrd W. Goerze C; AETORNEY ES Nov. 3, 1964 Filed Oct. 7, 1960 MAGNET FOR IMAGING PURPOSES 2 Sheets-Sheet 2 B-Gouss /d= aoo- 621 60oo l l l I l l I I I 25 20 l5 -l0 5 O 5 IO l5 DISTANCE FROM CENTER Mm Fig. 4

B- Gauss 960 l2 IQ 'l '11 l 'I/ L A II I I/ 840 l 800 Fig. 3

| l I l l Fig. 5

United States Patent 3,155,860 CQNTTNUUEELY VARIABLE PERMANENT MAGNET FQR IMAGING PURTQSESS Gerhard W. Goctze, Monroeviiie, Pan, assignor to Westinghonse Electric orporation, East Pittsburgh, Fan, in

corporaticn of llennsylvania Fiied Get. 7, 196b, Ser. No. 61,289 6 Qiaims. (QB. $13-84) This invention relates to permanent magnet systems for producing an adjustable homogeneous magnetic field which acts as a charged particle focusing medium. More particularly, this invention relates to permanent magnet focusing systems for focusing electron beams over relatively long paths as are characteristic of image tubes such as described in US. Patent 3,114,044 issued December 10, 1963 to E. I. Sternglass and assigned to the same assignee.

in the above copending application, a transmission secondary electron multiplier device is described. The device in the form of a light amplifier tube permits one to view dim radiation images directed onto an input electrode. The tube consists, in general, of a photoemissive input screen, a series of electron multiplying stages and a phosphor output screen. The resultant image appearing on the output phosphor is greatly enhanced in brightness over the input radiation image. in such a tube, it is found that it will operate at its optium only in a continuously variable homogeneous magnetic field which acts as an electron focusing medium along the entire path of the electrons from the input to the output.

It is found that the resolution of the tube with a magnetic focusing field is at least ten times greater than without it. Furthermore, there are times in the operation of the device wherein it is necessary to maximize the gain or the contrast or the best combination of these two features within the tube. It is therefore desirable to provide a variable magnetic field and yet maintain the field uniformity for such a tube in order to obtain optimum performance. An adjustable magnetic field may be achieved by an electromagnetic system but this method has the disadvantage that it requires an electrical power supply. Another disadvantage is that the electromagnet would have to be cooled if the tube were operated over a long period of time. it is also found that in order to provide the proper field configuration, an electromagnet focusing system results in an extremely bulky structure and therefore in the interest of conserving space a perma nent magnet system would be highly desirable. The disadvantages of known permanent magnet systems are that they normally do not provide a continuously variable uniform magnetic field and also an unobstructed view of the input and output ends of the tube.

It is accordingly an object of my invention to provide a permanent magnet focusing system which is continuously variable and maintains its uniformity.

It is another object to provide an improved permanent magnet system which allows an unobstructed view of the input and output ends of the tube.

t is another object to provide an improved permanent magnet focusing system.

It is another object to provide an improved permanent magnet focusing system to reduce space requirements therefor.

In an illustrative specific arrangement embodying the feature of this invention, there is employed a central or main annular magnet which substantially houses the electron tube and two smaller annular magnets which may be referred to as end pieces and which make it possible to continuously vary the magnetic field within certain limits without essentially changing the shape of the homogeneous field. The central magnet is magnetized axially in one 3,155,860 Patented Nov. 3, 1964 direction while the two end pieces are magnetized oppositely with respect to the central magnet. By moving the two end pieces relative to the central magnet and thereby varying the gap between the central magnet and the end pieces, one is able to adjust the magnetic field.

These and other objects are effected by my invention as will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts, and in which:

FIGURE 1 shows a side view of a focusing system which is an illustrative embodiment of the invention being used in conjunction with an image intensifier'tube;

FIGURE 2 is a schematic view illustrating the permanent magnet system shown in FIG. 1;

FIGURE 3 illustrates the image intensifier tube partly in section utilized in FIG. 1;

FIGURE 4 is a curve showing the magnetic induction as a function of distance from the center of the central magnet; and

FIGURE 5 is a curve illustrating the magnetic induction in the homogeneous part of the field as a function of the gap width D.

Referring in detail to FIGS. 1 and 3, an image intensifier type of electron discharge device 10 is shown. The tube 10 comprises a tubular envelope 12 closed at both ends. A suitable photosurface 14 capable of emitting electrons in response to a radiation image directed thereon is positioned at one end of the envelope 10. An output phosphor layer 16 is positioned at the opposite end of the envelope 1t? and emits light in response to electron bombardment. A plurality of planar dynode members 20 are positioned between the photosurface 14 and the output phosphor in to provide amplification of the electron image emitted by the photosurface 14. Suitable potentials are applied to the electrodes within the envelope 10 to accelerate the electrons emitted from the photosurface 14 due to radiation directed thereon into impingement with the facing surface of the adjacent dynode 20. A greater number of electrons are emitted from the 0pposite side of the dynode 29 which are accelerated and bombard the next dynode which in turn emits electrons of a greater number than that bombarding the opposite surface thereof. The electrons emitted by the last dynode 2d are acelerated and bombard the output phosphor 16 and generate a light image. A more complete description of the image intensifier tube is given in the abovementioned US. Patent 3,114,044.

For the application of the image intensifier tube it is desirable to have a magnetic field distribution to provide a uniform axial homogeneous field with the tube.

Referring more particularly to the magnetic structure illustrated in FIGS. 1 and 2, there is shown a permanent magnet assembly consisting of an elongated annular central or main core magnet 22 and two smaller annular magnets or ring magnets 24 and 26 which may be referred to as end pieces positioned at opposite ends of the central magnet 22. In the specific embodiment shown,

the outside diameter of the three annular magnets 22,

24 and 26 is 5.99 inches and the inside diameter of the three annular magnet members 22, 24 and 26 is 3.15 inches. If desired, the assembly of magnets 22, 24 and 26 may on the outside diameter taper from a maximum diameter at the center of the central magnet 22 to a minimum diameter at the exterior ends of the two end pieces 24 and 26. This will reduce the weight of the system and still provide substantially the same field. The length of the central magnet 22 is 5.67 inches and the length of the two outer end ring magnets 24 and 26 is 1.42 inches. The materials utilized for the permanent magnet members 22, 24 and 26 may be of any suitable type such as a ferromagnetic material such as ferrite. A

specific example is barium ferrite. The central magnet 22 is magnetized axially in one direction as indicated. The two end magnet rings 24 and as are also polarized axially but in an opposite direction to the main magnet 22 as indicated in FIG. 2.

The image intensifier tube 10 is mounted within the central magnet 22 and the central magnet 22 is or" substantially the same length as the image intensifier tube 10. Thus it can be seen that the axis of central magnet 22 is coaxial with the axis of the image intensifier tube ill. The central magnet 22 extends substantially the entire length of the electron path within the image intensifier tube 1t) between the photosurface lid and the output phosphor 16. The magnetic assembly is mounted on a horizontal base member 28 with two vertical planar support members 34 and 32 secured to the base member 28. The support members 39 and 32 are provided with an opening in which the central magnet 22 is positioned. The central magnet 22 is maintained in a fixed position by suitable fastening means. Another vertical support member 34 is provided on the base 23 and is spaced from one end of the central magnet 22. A corresponding support member 36 is provided on the base 28 and spaced from the opposite end of the central magnet 22. The vertical support members 3 and 36 are also secured to the base member 28. A plurality of longitudinal rods ll) are provided and secured to the support members F ll, 32, 3d and 36 to brace the support members. The suport members and rods are of a suitable non-n1agnetic material such as aluminum. The ring magnet 24 is mounted between the two vertical supports 30 and 3d. The other ring magnet 26 is positioned at the opposite end of the central magnet 22 between the support members 32 and 36.

The ring magnets 24 and 26 are mounted within the magnetic assembly so as to be axially movable toward and away from the ends of the main magnet 22. In the specific embodiment shown, the ring magnets 24 and 26 are provided with annular support rings 42 and 44 respectively around their periphery in which threaded apertures are provided. It is desirable to provide at least three threaded apertures in each support ring 42 and as; spaced at about 120 degrees. Longitudinal rod members 4-6 pass through these openings and are supported in the support members 30, 32, 34 and 36 and are fixed thereto to prevent longitudinal movement. The rod members 46 are free to rotate. The rod members 46 are provided with external threads thereon engaging the threaded apertures in the support rings 42 and 44. One of the rods 6 is provided with a knob 51 for rotating the rod member 46. A gear wheel 52 which is provided on each of the rod members 46 and each of the gear wheels 52 is connected by a chain 54. By turning the knob 50 the rods 46 rotate and cause the ring magnets 24 and 26 to move axially either in toward the main magnet 22 or away from the main magnet 22.

By varying the gap distance between the end ring magnets 24 and 26 and the central magnet 22, the magnetic field may be varied as illustrated in FIG. 4. The upper curve 60 is taken with the gap widths betwen the central magnet 22 and the two ring magnets 24 and 26 equal to approximately zero while the lower curve 62 was taken with the gap width approaching infinity. Therefore, one is able to obtain 'a field variation between the two curves as illustrated by varying the spacing between the ring magnets 24 and 26 and the central magnet 22. This variation is more clearly represented by FIG. in which the curve 64- illustrates the variation in the magnetic induction as a function of the gap width in millimeters.

As is indicated, by the utilization of a central magnet 22 polarized on one direction and two end magnets 24 and 26 polarized in the opposite direction with respect to the central magnet, one is able to obtain a homogeneous magnetic field which is easily adjustable and yet still provide a continuously variable homogeneous uniform magnetic field.

Although the present invention has been shown in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various changes and modifications Without departing from the spirit and scope thereof.

1 claim as my invention:

1. An electron tube comprising a collector electrode and means for projecting an electron beam along a path toward said collector, an elongated cylindrical permanent magnet disposed coaxially with respect to the path of said electron beam and extending for substantially the length of the path of said electron beam, said cylindrical member being magnetized in a direction parallel to said electron beam path, a first ring member disposed coaxially with respect to said electron path at one end of said cylindrical member, said first ring member being magnetized in a direction parallel to said electron beam but in an opposite direction with respect to the magnetization of said cylindrical member and a second ring member disposed coaxially with respect to said electron path and positioned at the opposite end of said cylindircal member with respect to said first ring member, said second ring member magnetized in a direction parallel to said electron beam path and oppositely magnetized with respect to the cylindrical member.

2. A magnet arrangement for focusing an electron beam within an electron tube generated by an electron source and collected by a collector electrode comprising a hollow elongated annular permanent magnet surrounding said tube from said electron source to said collector electrode, said elongated magnet being magnetized in a direction parallel to the axis of said electron beam, a first short annular permanent magnet coaxial with said elongated annular magnet and spaced axially from one end thereof, said first short annular member being magnetized in a direction parallel to said electron beam and oppositely with respect to said elongated magnet, and a second short annular magnet coaxial with said elongated annular magnet and spaced axially from the opposite end of said elonga ed magnet with respect to said first short annular magnet, said second short annular magnet magnetized in a direction parallel to the path of said electron beam and oppositely magnetized with respect to said elongated magnet and mechanical means for adjusting the spacing between said first and second short annular magnets with respect to said elongated magnet to vary the field within said magnetic system.

3. A permanent magnet arrangement for focusing an electron beam over a distance which is comparatively long with respect to the diameter of the electron beam, said arrangement comprising three annular coaxial permanent magnet members, said permanent magnet members spaced apart axially and having substantially the same inside diameter, the center magnet of said three magnets elongated and having a length comparative to the path of said electron beam, the length of said center magnet being greater than the inside diameter of said annular members and the two outer annular members having a length which is less than the inside diameter of said annular members.

4. A magnet structure for focusing an elongated electron beam comprised of first, second and third coaxial annular permanent magnets, said first magnet surrounding said beam and extending for the length of said beam to produce a longitudinal magnetic field extending along the trajectory axis of said beam, said first magnet magnetized longitudinally in a first direction, second magnet positioned at one end of said first magnet and polarized longitudinally in an opposite direction with respect to said first magnet and said third magnet positioned at the opposite end of said first magnet with respect to said second magnet and magnetized longitudinally in a direction opposite with respect to said first magnet.

5. A cathode ray tube comprising an electron source for producing a beam of electrons, a target member for said beam of electrons, a first focusing permanent magnet 5. positioned parallel to the path of said electron beam for producing a longitudinally magnetic field extending along the trajectory axis of said beam, said first magnet extending over substantially the entire length of said electron path, said first magnet being magnetized parallel to the path of said electron beam, a second focusing permanent magnet positioned at one end of said first magnet and magnetized in a direction opposite With respect to said first magnet and a third permanent focusing magnet positioned at the opposite end of said first magnet with respect to said second magnet and oppositely magnetized with respect to said first magnet and means for varying the spacing between said first magnet and said second magnet and the spacing between said first magnet and said third magnet to vary the magnetic field Within the magnetic 15 structure.

6. A magnetic electron lens comprising three coaxial spaced annular permanent magnets, said magnetic members magnetized to produce like polarity poles on facing surfaces, said magnetic members having the same inner diameter and the outer diameter of said members tapering from a maximum at the center of the inner member to a minimum at the outer ends of the outer members, said inner magnetic member benig elongated so that its length is greater than the inner diameter of said annular magnetic members and said tWo outer magnetic members having a length less than the inner diameter of said annular magnetic members.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRON TUBE COMPRISING A COLLECTOR ELECTRODE AND MEANS FOR PROJECTING AN ELECTRON BEAM ALONG A PATH TOWARD SAID COLLECTOR, AN ELONGATED CYLINDRICAL PERMANENT MAGNET DISPOSED COAXIALLY WITH RESPECT TO THE PATH OF SAID ELECTRON BEAM AND EXTENDING FOR SUBSTANTIALLY THE LENGTH OF THE PATH OF SAID ELECTRON BEAM, SAID CYLINDRICAL MEMBER BEING MAGNETIZED IN A DIRECTION PARALLEL TO SAID ELECTRON BEAM PATH, A FIRST RING MEMBER DISPOSED COAXIALLY WITH RESPECT TO SAID ELECTRON PATH AT ONE END OF SAID CYLINDRICAL MEMBER, SAID FIRST RING MEMBER BEING MAGNETIZED IN A DIRECTION PARALLEL TO SAID ELECTRON BEAM BUT IN AN OPPOSITE DIRECTION WITH RESPECT TO THE MAGNETIZATION OF SAID CYLINDRICAL MEMBER AND A SECOND RING MEMBER DISPOSED COAXIALLY WITH RESPECT TO SAID ELECTRON PATH AND POSITIONED AT THE OPPOSITE END OF SAID CYLINDIRCAL MEMBER WITH RESPECT TO SAID FIRST RING MEMBER, SAID SECOND RING MEMBER MAG NETIZED IN A DIRECTION PARALLEL TO SAID ELECTRON BEAM PATH AND OPPOSITELY MAGNETIZED WITH RESPECT TO THE CYLINDRICAL MEMBER. 